I just go with tire pressure recommendations on the side of the tires which is usually 32-35, 35 being max pressure with max payload. We don't use the X5 for long trips. I inflate the tires on the X5M with luggage etc. in the back, which, I'm guessing, weighs about 500 pounds, to 35 psi. But, that is trying to increase mileage not due to payload. I've tried higher pressure but the ride is compromised too much for a long trip.

__________________
X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas